WIP: (testing, CI) Run up to 4 gauss jobs at once in run-gauss CI test

The run-gauss test can take a long time to complete as Gauss is run for each decfile sequentially. I opened this MR to slightly modify the test to allow up to 4 Gauss processes (can be tuned to suit) to be run in parallel at once. There may be reasons I'm not aware of that mean this is not a good idea (apologies if this is the case.)

Examples:

• when one or more decfiles fail: https://gitlab.cern.ch/lhcb-datapkg/Gen/DecFiles/-/jobs/12752673
• when all decfiles pass: https://gitlab.cern.ch/lhcb-datapkg/Gen/DecFiles/-/jobs/12753189

For comparison with old test:

• when one or more decfiles fail: https://gitlab.cern.ch/lhcb-datapkg/Gen/DecFiles/-/jobs/12750344
• when all decfiles pass: https://gitlab.cern.ch/lhcb-datapkg/Gen/DecFiles/-/jobs/12752226

Todo:

• test scenario where one or more of the decfiles fail
• test scenario where all the decfiles pass
• remove test decfiles included in this MR before marking ready
• rebase + squash